High frequency and high intensity exercise can triple a woman's chances of experiencing fertility problems, say Norwegian researchers. Women who exercise every day or at such an extreme intensity that they become physically exhausted, have less chance of getting pregnant in the first year of trying than women who exercise 'moderately', says Dr Sigridur Gudmundsdottir, who led the research at the Norwegian University of Science and Technology.

'We believe it is likely that physical activity at a very high or very low level has a negative effect on fertility, while moderate activity is beneficial', said Dr Gudmundsdottir. The study involved 3000 women in Norway who were questioned about the frequency, duration and intensity of their fitness regimes between 1984 and 1986, and then asked about their pregnancies ten years later. 'Among these women, we found two groups who experienced an increased risk of infertility. There were those who trained almost every day and there were those who trained until they were exhausted. Those who did both had the highest risk of infertility', said Dr Gudmundsdottir, whose research appears in the medical journal Human Reproduction.

Gudmundsdottir and her team discovered that the women under 30 who exercised the most were those who had experienced the most problems conceiving; a quarter of these women were unable to conceive in the first year of trying, compared to a national average of seven per cent.

The effect of the extreme exercise did not last however, 'The vast majority of women in the study had children in the end', Gudmundsdottir said, adding 'and those who trained the hardest in the middle of the 1980s were actually among those who had the most children in the 1990s'.

It is known that elite female athletes can experience fertility problems, however, this study shows that women who push themselves in their own exercise regimes are at risk too. Experts believe that intense physical activity, such as gruelling physical workouts, can actually leave the body energy-deficient, and unable to maintain all of the required hormonal mechanisms required for successful fertilisation. Some are warning however that the study only shows an association between extreme physical activity and fertility problems, not a cause-and-effect relationship, and the results should be seen in the context of other research recommending that healthy women benefit most from moderate exercise.